Selectins are a family of glycoproteins that initiate adhesion of leukocytes to platelets or endothelial cells by Ca2+-dependent interaction between the lectin domain of the selectin and oligosaccharides on target cells. Selectin-mediated adhesion is required for the movement of inflammatory cells into sites of inflammation. P-selectin is a receptor for myeloid cells expressed on activated endothelium and platelets that binds with low affinity to sialylated, fucosylated, lactosaminoglycans including sialyl Lewis x (NeuAca2-3Galbl-4[Fucal-3]GlcNAc). However, we have recently discovered a high affinity glycoprotein ligand for P- selectin on myeloid cells which may be a potentially important physiologic mediator of selectin-mediated leukocyte adhesion. The ligand, called P- selectin Glycoprotein Ligand 1 (PSGL-1), is a mucin-like glycoprotein composed of two 120 kDa disulfide-linked subunits expressed in neutrophils. Recognition of PSGL-1 by P-selectin is Ca2+-dependent and requires sialylated O-linked oliosaccharides on its polypeptide backbone. We found that subsets of CD4+, CD8+, and CD16+ lymphocytes bound P- selectin in a Ca2+-dependent fashion. Binding was abolished by treatment of cells with protease or sialidase, indicating that lymphocytes express one or more sialoglycoprotein ligands for P-selectin. We have identified a 120 kDa P-selectin ligand from purified T cells. Recognition of the T cell ligand by P-selectin was Ca2+-dependent and was abolished by sialidase treatment of the ligand. However; in contrast to PSGL-1, recognition of P-selectin by the T cell ligand requires N-linked oligosaccharides on its polypeptide backbone. Although the relationship between this ligand and PSGL-1 is unknown, we postulate that the oligosaccharides required for P-selectin recognition by the T cell ligand and PSGL-1 are distinct. We propose to: l) isolate the T cell ligand from normal T cells by P-selectin affinity chromatography; 2) determine the relationship between the T cell ligand and PSGL-1 polypeptide backbones by determining their relative size after deglycosylation and their immunologic relatedness; 3) characterize the glycosylation of the ligand from normal T cells using a combination of lectin affinity chromatography and glycosidase susceptibility; and 4) analyze the structure of the oligosaccharides derived from the ligand isolated from T cell clones metabolically labeled with [3H]monosaccharides. These studies should provide new insights into the molecular mechanisms for P-selectin interaction with lymphoid cells and the potential role of P-selectin in the recruitment of lymphoid cells into sites of inflammation.